1. Field of the Invention
The present invention relates to an electro-magnetically operated valve and, in particular, to a two-coil solenoid valve (TCV) for use in an internal combustion (i.c.) engine fuel injection system.
High-pressure, electronically controlled i.c. engine fuel injection systems require valves offering fast and precise switching between fuel flow ports. Poppet valves which have typically been employed in fuel injection systems do not generally offer sufficiently fast switching rates for modern fuel injection systems. Also, poppet valves are known to suffer from "valve bounce", whereby the valve member on moving to a switched position momentarily rebounds from its end stop. The bouncing of the valve member can adversely affect control of fluid amounts passing through the valve and this can be critical to improving the performance of i.c. engines to increase fuel consumption efficiency and reduce undesirable exhaust emissions.
It is therefore becoming increasingly common to employ TCV's in i.c. engine fuel injection systems. A typical TCV comprises a metallic spool valve member mounted in a housing for axial movement between two pole pieces. Solenoid coils located in the pole pieces are selectively controlled to effect movement of the spool valve member between the two pole pieces. The spool valve member may have axially extending bores formed therein or radially extending grooves formed in its outer surface which afford communication between selected ports of the valve dependent on the controlled position of the valve member within the housing. U.S. Pat. No. 4,168,688 discloses a spool valve type TCV which has a hollow spool valve member.
Magnetic remanence (or residual magnetism) in the material of the pole pieces is often regarded as a problem to be overcome in electro-magnetically operated valves since it provides a resistance to movement of the valve member away from a pole piece. The "capture", of the valve member at rest by the magnetic remanence of a pole piece to which it is adjacent can lead to undesirable switching delays. However, as disclosed in U.S. Pat. No. 3,743,898, the magnetic remanence of the pole pieces can be put to good effect in electro-magnetically operated valves where it is desired to limit electrical energy consumption, for example in low-voltage systems such as vehicle battery systems. The magnetic remanence can be employed to latch the valve member to a pole piece when the valve member is at rest after having been caused to switch position. Thus, when the valve member is at rest, the valve does not consume electrical energy. The use of magnetic remanence in this way can also negate the need to incorporate biasing means such as springs in the valve thus simplifying the valve's structure.
A minimum level of remanence in the pole pieces is necessary in order to latch the valve member after it has moved and an energised coil that caused it to move has been de-energised, but it is equally important that a maximum desired level of remanence in the pole pieces is not exceeded. Should the maximum desired level of remanence in the pole pieces be exceeded, this can affect the performance of the valve in two ways. Firstly, the allowable peak electrical current in a coil may not produce sufficient pull on the valve member to remove it from its latched position and an increase in the coil magnetising current may cause heat dissipation and other problems. Secondly, an excessive level of remanence will distort the timing performance of the valve. It becomes increasingly difficult, with increasing levels of remanence, to determine the release time of the valve member from the pole piece to which it is latched where the pulling force of the attracting coil is not substantially greater than the latching force exerted by the latching pole piece.
A known method of reducing the magnetic remanent force acting on the valve member is to reduce contact between the valve member and an adjacent pole piece. This can be achieved by providing a stepped portion on either or both the valve member and pole piece such that the area of contact between the valve member and the pole piece is minimised and the valve member is effectively held at a distance from the pole piece.
A problem particularly relevant to the use of TCV's in i.c. engine fuel injection systems, however, is the detrimental effects of wear between the ends of the valve member which contact the pole pieces and the parts of the pole pieces which act as end stops for the valve member. Ordinarily, the metallic material of the pole pieces and the valve member is relatively magnetically soft in order to control the level of magnetic remanence in the pole pieces and to a lesser degree in the valve member, but this material tends to also be mechanically soft and is therefore susceptible to wear. Wear caused by the repeated hammering of the valve member on the pole pieces will distort the operation efficiency of the valve. In particular, when the valve member and/or the pole pieces are provided with a stepped portion to reduce contact therebetween, the repeated hammering of the valve member on the pole pieces caused by the high speed, high frequency operation of the valve will lead to plastic deformation of the stepped portion and also possibly of the valve member and pole pieces and will allow the valve member to move into closer contact than intended with the pole pieces. This will affect the switching precision of the valve and will cause the valve member to be more firmly latched to the pole pieces thus requiring greater magnetising coil currents to effect its removal from the pole pieces on switching.
2. Description of the Prior Art
The abovementioned problem has been addressed in U.S. Pat. No. 5,488,340 in which the compositions and treatment of the metallic materials of the valve member and the pole pieces are specified in order to provide a material of increased mechanical hardness so as to reduce its susceptibility to wear. However, whilst the solution proposed in U.S. Pat. No. 5,488,340 goes some way to addressing the problem of wear, it fails to provide a means of ensuring that the level of magnetic remanence in the pole pieces remains generally constant over the anticipated life of the valve.